Electrically driven hammer drills are known in which a motor causes a driving member in the form of a flying mass to be reciprocally driven in a hollow spindle by means of piston, while at the same time causing the piston to rotate about its longitudinal axis. The spindle supports a working member such as a drill bit, either by direct mounting of the drill bit to the spindle, or by supporting a tool holder which in turn supports the drill bit, such that rotation of the spindle about its axis causes rotation of the drill bit to effect a drilling action, while reciprocating movement of the flying mass within the spindle imparts hammer impacts to the drill bit.
The spindle of a hammer drill of such known types is generally manufactured in a single piece. As a result, the entire spindle generally needs to be constructed as robustly as that part of the spindle subjected to the greatest amount of wear, even though most of the spindle does not need to be constructed to such a level of robustness. This significantly increases the cost and difficulty of manufacturing the hammer drill. Furthermore, when the hammer drill is repaired, the entire spindle generally needs to be replaced, even if only part of the spindle is damaged, which makes repair of the drill costly.
U.S. Pat. No. 5,373,905 discloses a hammer drill in which a spindle in which a striker mass is reciprocally driven is formed from two separate parts, i.e. a guide tube, and a tool socket arranged forwardly of the guide tube, the guide tube and tool socket overlapping each other in a certain region and being connected to each other by means of a press fit. As a result, only the tool socket needs to be formed from a robust, high-grade material, which significantly reduces the cost of manufacture and repair of the drill.
However, this arrangement suffers from the drawbacks that because the guide tube and tool socket are connected to each other by means of a press fit, the cooperating parts of the guide tube and tool socket must be manufactured to accurate dimensions, as a result if which manufacture of the drill is still costly. Also, the hammering action of the striker mass against the tool socket causes the tool socket to tend to detach from the guide sleeve, as a result of which components such as profiled rings securing the guide tube and tool socket together are subjected to significant stresses and can become damaged and must be frequently replaced.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.